Re: [PATCH] KVM: hyper-v: Add new exit reason HYPERV_OVERLAY

[Paolo Bonzini]

On 23/04/21 11:58, Alexander Graf wrote:
> > In theory userspace doesn't know how KVM wishes to implement the
> > hypercall page, especially if Xen hypercalls are enabled as well.
>
> I'm not sure I agree with that sentiment :). User space is the one that 
> sets the xen compat mode. All we need to do is declare the ORing as part 
> of the KVM ABI. Which we effectively are doing already, because it's 
> part of the ABI to the guest, no?

Good point.  But it may change in the future based on KVM_ENABLE_CAP or 
whatever, and duplicating code between userspace and kernel is ugly.  We 
already have too many unwritten conventions around CPUID, MSRs, get/set 
state ioctls, etc.

That said, this definitely tilts the balance against adding an ioctl to 
write the hypercall page contents.  Userspace can either use the 
KVM_SET_MSR or assemble it on its own, and one of the two should be okay.

Paolo

> > But userspace has two plausible ways to get the page contents:
> >
> > 1) add a ioctl to write the hypercall page contents to an arbitrary
> > userspace address
> >
> > 2) after userspace updates the memslots to add the overlay page at the
> > right place, use KVM_SET_MSR from userspace (which won't be filtered
> > because it's host initiated)
> >
> > The second has the advantage of not needing any new code at all, but
> > it's a bit more ugly.
>
> The more of all of that hyper-v code we can have live in user space, the 
> happier I am :).